1. Field of the Invention
The present invention relates to a recording apparatus, and particularly a recording apparatus in which shading correction processing can be performed.
2. Description of the Prior Art
In the widespread use of computers and communication apparatus, recording apparatus outputting information of these apparatus by enabling recording heads to form digitized dots has been generally used. In addition, such digitized recording apparatus is generally applied into copy machines. In a recording apparatus using recording heads, in order to increase the recording speed, it is a general habit to use a multi-head including a plurality of recording elements. However, it is rather difficult to fabricate a plurality of recording elements in an individual multi-head in a uniform quality and hence, the characteristic of fabricated recording elements may not be stabilized. As a result, shading or density shading (density ununiformity of an image which is recorded on a recording medium by the reading head which has a plurality of recording elements) occurs in the recorded image which causes the reduction of the image quality. By repetitive use of recording elements, recording elements suffer from aged deterioration which also causes characteristic instability and shading.
In order to solve above problems, what is proposed is a method for correcting characteristic of recording elements by means that a specific read-out part for reading out shading at an arbitrary time is placed in the recording apparatus and shading correction data are generated according to the read-out data.
FIG. 1 shows a diagrammatic picture illustrating an example of such a method for reading out shading as described above.
In FIG. 1, a reference numeral 121 is a recording sheet, 122 a recording head, 123 a recording element placed in the recording head 122, 124 a read-out head composed of CCD, 125 a read-out element installed in the read-out head 124, and 126 a test pattern obtained by scanning in the X direction the recording head 122 including recording elements 123 which are arranged in the Y direction relative to the recording sheet in order to record one line pattern. The number of the read-out elements in the read-out head 124 is equivalent to that of the recording elements of the recording head 122. By scanning the read out head 124 in the direction of an arrow B in FIG. 1, the density of the pattern 126 is read out. In this configuration shown in FIG. 1, the number of density data read-out by each read-out element 125 in a single scanning operation is equal to the number of the recording elements 123 of the recording head 122, and the average of these density data is used as an ideal density to be realized by individual recording elements.
Even if input signals to all the recording elements 123 of the recording head 122 are identical to one another, in case that the read out density has shading property, the input signals should be corrected. For example, with respect to the recording element giving lower density, the input signal is corrected so as to be larger, and with respect to the recording element giving higher density, the input signal is corrected so as to be smaller. So far, the density defined by individual recording elements can be corrected to be uniform. In case that shading occurs as the recording apparatus is used, further shading correction is performed in order to establish uniform density. The modification of input signal values described above is performed by referring conversion tables.
By referring to FIGS. 2 and 3, an outline process of the shading correction processing is described below.
Now assume that the relationship between the input (driving) signal to a certain recording element n and the density of the recorded (outputted) image or dot is one shown in FIG. 2. It can be stated that the recording element n recording an image with the density OD.sub.n with respect to the input signal S. If the average density over all the recording elements with respect to the driving signal S is assumed to be OD, in other words, the correction density is assumed to be OD, the recording element n records an image with higher density. In order to correct the density of the recording element n from OD.sub.n to OD, the intensity of the input signal to the recording element n is modified from S to S' by referring to the conversion table.
FIG. 3 is a graph illustrating a content of the conversion table. The table shown in FIG. 3 contains 64 correction curves or straight lines, each of which corresponds to a couple of an input signal S and its corresponding output signal, each signal formed in 255 gray level data. In FIG. 3, only two out of 64 lines, A and B, are shown. Information about which correction curve is selected to an individual recording element are separately stored and referred in responsive to the read-out density data in order to select a desirable correction curve. When the input signal S is inputted with respect to one recording element, this element giving the density according to the correction curve or line selected. For example, with respect to the recording element accepting the input signal S and outputting the density OD.sub.n, correction line B is selected and input signal is modified to S' so that the density recorded by that recording element is OD.
The density distribution established in the configuration defined as in FIG. 1 is generally found to be one shown in FIG. 4, where the horizontal axis represents the position of recording elements in the recording head, and the vertical axis represents the recording density defined by individual recording elements. One problem in this situation is that the density by the recording elements at the end parts of the array of recording elements is different from the density by the recording elements at the rest part of the array. That is, a pixel recorded by the recording elements at the rest part of the array involves recorded parts by the adjacent recording elements, on the other hand, the pixel recorded by the recording elements of the end parts of the array involves a part of ground of the recording sheet. Therefore, in the case that the sheet color is white, as shown in FIG. 4, the density at the edge parts is formed n a gradually increasing or decreasing curve in which the measured density is estimated to be less than the actual density. If the density correction is performed in such a situation, the density at the connection parts between the recording lines repetitively developed by multiple scanning operations of the recording head may be modified to be greater than the actual density.
In order to solve the above problem, as described in U.S. Ser. No. 07/593,765 (filed on Oct. 4, 1990) and U.S. Ser. No. 07/711,648 (filed on Jun. 11, 1991), there is a correction method in which three lines (three time scan operations) are recorded and only the central line data are used for correction calculations. In the case of recording three lines, recording elements at the both end parts of the array of recording elements form pixels so as to be adjacent to each other so that the above described problem may be solved.
In either method for density shading correction, it is known that several problems specific to text-pattern read-out procedures still exist.
(1) The first problem relates to the density shading correction in recording images by using a multi-head, for example, in reduction recording in copy machines.
As for a method for reduction recording, what is well known is a method that, by selecting input signals defined to individual recording elements, recording is performed not by all the recording elements but by partial recording elements. This method is further categorized into two methods. Examples on these methods applied to reduction recording in the recording apparatus shown in FIG. 5 are described below.
FIG. 5 is an isometric view of the main part of the recording apparatus using an ink jet recording method. In FIG. 5, the recording head 4 has a plurality of orifices for ejecting ink fluids in an array extended in Y direction ejecting ink fluids corresponding to individual orifices. The recording head 4 is guided by the guide shaft 5 and scanned in X direction in the figure, and in response to this transport movement, the recording head ejects ink fluids and forms dots on the recording sheet 2. By the single scan movement of the recording head, one line recording is established. The recording sheet 2 is fed in Y direction by the feed roller 1 driven by the motor not shown, which establishes a plurality of line recordings continuously. The paper press board 3 is installed near the recording area developed by the recording head 4 in order to make the recording area on the recording sheet 2 flat.
In the first method of reduction recording, the rotational movement of the feel roller 1 is controlled in an ordinary manner, and the scan operation of the recording head 4 is performed several times in responsive to a single rotational movement of the feed roller 1. At each scan operation, orifices used for recording operation are altered by blocks. This means that, in a multi-head having N orifices, in the first scan, orifices from the edge to n1 are used, and in the second scan, orifices from (n1+1) to n2 are used, and in the K-th scan, orifices from n(k-1)+1 to N are used for recording, respectively. Owing to this sequence, reduction recording with 1/k magnification can be performed. After terminating the recording at the K-th scan, the feed roller 1 is rotated in order to move the recording sheet in the transport distance equivalent to the N pitches of orifices. So far, reduction recording is repeated in the same manner.
In the second method of reduction recording; the rotational movement of the feed roller 1 and the scan operation of the recording head 4 are altered mutually, or the rotational angles of the feed roller 1 which determines the transport displacement of the recording sheet 2 is controlled in responsive to the recording magnification. For example, in recording by using L(L.ltoreq.N) orifices in the multi-head having N orifices, reduction recording with L/N magnification can be performed, in which the transport displacement of the recording sheet is equivalent to the L pitches of orifices.
In the above two methods, the number of reduction patterns obtained by the second method is greater than that obtained by the first method. In the first method, as recording is performed by K times sheet feed operations, in each time of which N orifices are used, K must be a divisor of N and the number of reduction patterns is limited to the number of divisors of N. On the other hand, in the second method only if the condition, L.ltoreq.N, is satisfied, the number of reduction patterns is theoretically taken to e the number of orifices. This is the reason why the second method is usually used for reduction recording.
However, even in the above described method for reduction recording, if the conventional method for density shading correction is adopted, the following problems exist.
As described above, the density of a specific pixel to be read is affected the density of pixels adjacent to the specific pixel. In the example of a multi-head having n orifices (recording elements), the correction data for the i-th recording element includes the effect by the (i-1)th and (i+1)th recording elements. In other words, the correction data for the i-th recording element is most effective when a pixel is recorded by the i-th recording element between pixels defined by the (i-1)th and (i+1)th recording elements.
FIGS. 6A and 6B are recorded pixels and density distributions in an ordinary recording condition without density shading correction and with density shading correction, respectively. Without density shading correction as shown in FIG. 6A, density shading is found to be to a relatively large extent. In contrast, in the example shown in FIG. 6B, by altering the number of pixels at individual recording elements instead of varying the values of input signals, density shading in a designated region can be reduced.
However, in case of reduction recording by using orifices from 1 to i out of n orifices, recording is performed in the following manner.
At first, the recording head is moved in X direction in FIG. 5 while recording one line using recording elements from 1 to i. Next, at the time when the recording head is moved back to the home position, the recording sheet is moved in Y direction by the i pitches of recording elements which is i/n of an ordinary transport displacement of the recording sheet. And one line is recorded by using recording elements from 1 to i. In recorded images in an ordinary magnification ratio, pixels adjacent to the pixel defined by the i-th recording element are those defined by the (i-1)th and (i+1)th recording ratio, as the i-th recording element is defined as a recording element at the edge part, the pixel defined by this i-th recording element is located between the (i-1)th recording element and the 1st recording element. Recorded pixels and density distribution in reduction recording after density shading correction are shown in FIG. 6C. Density shading correction applied in FIG. 6B is also applied to individual recording elements in reduction recording in FIG. 6C. In this case shown in FIG. 6C, if density shading correction is performed without considering the mutual effect between recording elements "i" and "1" which form a connection part of recorded images, density shading correction is not sufficient enough and density shading may be contained in the density distribution which is found to be a black or white line noise in a recorded image. In the prior art, due to above described density shading, there is a problem that the quality of reduced recorded images is worse than that of ordinary recorded images.
(2) The second problem relates to above described density shading in a recording apparatus for recording images with a plurality of different ink colors.
In reading out a test pattern recorded with a plurality of different ink colors, the density levels of individual ink colors are generally different from one another. Therefore, the levels of density shading found in read-out data changes in every individual color image. If an identical density shading correction is applied commonly to individual ink colors, density shading for specific color tones may not resolved.
In order to solve this problem, a method in which a distinctive correction procedures is applied to a specific color tone is possible. However, this method requires a complex apparatus structure and control process which may lead to another new problem.
This problem is not specific to the recording apparatus using an ink jet recording method but found in a recording apparatus using a plurality of recording elements and a plurality of ink colors, for example, a thermal printer.
(3) The third problem is that, in case of using an exchangeable recording head, density shading correction may give a bad effect dependent of the characteristics of the recording head.
(4) The fourth problem is that, in case that a test pattern chart is not placed in a proper position the read-out data of the test pattern cannot be obtained precisely.
(5) The fifth problem is that, in case that a test pattern is not recorded in a good condition on the test pattern chart, the read-out data of the test pattern cannot be obtained precisely.